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Space is largely empty; this is a crucial point I will come back to. Assume a spherical distribution of (dark) matter, with density small enough that we can ig
by aroberge 12y ago
Space is largely empty; this is a crucial point I will come back to.
Assume a spherical distribution of (dark) matter, with density small enough that we can ignore relativistic effects (which is almost always the case). If you are inside the spherical distribution at a distance R from the center of that distribution, the only effect you will feel is that of the mass inside a sphere of radius R, centered on that distribution. (This is well known for any inverse square law; I can explain more if required.)
The average density of normal/dark matter is very, very small. Inside our solar system, the dominant gravitational effect is that of the Sun: any dark matter having the expected density (from other measurements) would contribute only a tiny amount, so as to be essentially undetectable.
On much, much larger scales, such as the scale of galaxies, far enough from the center (and assuming the dark matter is essentially a spherical distribution coinciding with the galaxy), the cumulative amount of the additional gravitational pull is enough to be detected. Remember that the distance to the nearest stars is huge. Yet, our Sun is within a galaxy. So, as I wrote above, the average density of normal/dark matter is very, very small. However, over such large distances as those of the size of galaxies, the cumulative effects can be observed. However, distances involved within our solar system are such that no additional pull from dark matter (comparing the motion of say Mercury to that of Jupiter) is expected to have a visible effect.
- netcraft 12y agois it possible that dark matter is regular matter, just so diffuse that we don't notice/see it? could it be hydrogen atoms just floating in empty space, at such low densities we would never notice but in the grand scheme of things add up? Edit: I dont mean to propose a potential solution here, it was more a question of why it must be unknown matter, why can't it be normal matter.
- aroberge 12y agoNo, it is not possible. Cosmological models are actually tightly constrained by data. Even in the late 80s, when we did not have such good data, I remember being amazed at the fact that primordial nucleosynthesis calculations together with observational data were enough to conclude that the number of light "normal" neutrino species was 3, while there were still speculations in Particle Physics that we could have more than 3 generations of particles. The data accumulated over the last 30 years has constrained things even more. If some other explanation than invoking some new type of matter that can not be seen was possible to explain the observational data, don't you think that Physicists would use that instead of coming up with postulating that the Universe is dominated by "dark matter" and "dark energy" which we have not seen yet?
- netcraft 12y agoLooking back at what I wrote I can understand your reaction - im wasnt trying to propose a new solution that someone hadn't considered. It was more a question of why couldn't it be that, which you answered quite well, thank you.
- BaryonBundle 12y agoNeutral hydrogen in space is actually quite readily observed via radio telescopes as the famous spin-flip 21cm line. This signal has been very useful in mapping the distribution of neutral hydrogen within our own galaxy and those of our galactic neighbors. In addition to this, there exist many arguments in favor of classifying normal matter and dark matter separately.
- lisper 12y agoThanks for that explanation. Two followup questions if you don't mind: 1) Why doesn't dark mater clump together to form dark galaxies? 2) Do we see dark matter's influence in the motions of the stars in the milky way?
- inclemnet 12y ago> Why doesn't dark mater clump together to form dark galaxies? Because of the weak nature of its interactions - it barely interacts with itself, never mind binding into local blobs. You can imagine starting with a big sphere of dark matter; under gravity it will collapse inwards towards its centre, but although it falls to a small region it just passes through itself and goes right out the other side again...oscillating right back to something close to the initial sphere. This is in contrast to 'normal' matter, which strongly interacts with itself and so will not pass through itself - instead it can bind and (through a range of physical effects) become planets, stars, or other things. Or at least heat up and redistribute energy in more interesting ways. > Do we see dark matter's influence in the motions of the stars in the milky way? Yes, you can see e.g. https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dark_matter_halo https://en.wikipedia.org/wiki/Dark_matter_halo#Milky_Way_dar... for some information.
- lisper 12y agoThanks!